Nonlinear Finite Element Analysis of Double-Skin Steel Tube High-Strength Concrete Axially Compressed Short Columns

Literature Overview

The paper by Peng Yingyi, Zheng Aiguo, and Tan Kefeng (2010), published in the Journal of Southwest University of Science and Technology, Vol. 25, No. 4, pp. 18-22, presents a nonlinear finite element (FE) analysis of double-skin steel tube high-strength concrete (DST-HSC) axially compressed short columns. The authors, from Southwest University of Science and Technology's School of Civil Engineering and Architecture and the Deyang Railway Sleepers Factory, were supported by the National Natural Science Foundation of China (Grant No. 59338120). The study employs the ANSYS finite element software to simulate the behavior of DST-HSC columns and compares the numerical results with experimental data, demonstrating good agreement between the load-displacement curves and providing insights into the load-bearing and failure mechanisms.

Core Technical Content

The double-skin steel tube (DST) configuration consists of two concentric steel tubes with a concrete core confined between them. The inner tube provides direct confinement to the concrete, while the outer tube provides additional confinement through the intertube space, which may be filled with air, grout, or additional concrete. This dual-confinement mechanism significantly enhances the load-bearing capacity and deformation capacity of the column compared to single-skin CFST columns.

The FE model was developed using ANSYS with appropriate material models for the steel tubes and high-strength concrete. The nonlinear analysis accounted for:

1. Geometric nonlinearity: Large deformations and changes in geometry during loading
2. Material nonlinearity: Plastic behavior of steel and nonlinear stress-strain relationship of concrete
3. Contact nonlinearity: Contact and separation between the inner tube and concrete, and between the outer tube and the intertube material

Numerical Simulation and Results

The load-displacement curves obtained from the FE analysis showed good agreement with the experimental results, validating the accuracy of the numerical model. The stress contour plots of the longitudinal stress distribution revealed the load-bearing mechanism of the DST column:

• Inner tube: Bears the primary confinement pressure from the concrete core and transfers it to the outer tube through the intertube medium.
• Outer tube: Provides secondary confinement and contributes directly to the axial load resistance.
• Concrete core: Experiences enhanced triaxial compression due to the dual confinement, leading to increased strength and ductility.

The stress distribution analysis demonstrated that the dual confinement mechanism of the DST configuration leads to a more uniform stress distribution in the concrete core compared to single-skin CFST columns. This uniformity reduces the likelihood of localized concrete crushing and promotes a more progressive failure mode.

Engineering Practice Integration

The DST-HSC column represents an advanced structural system with significant potential for applications requiring high strength, ductility, and fire resistance. The FE analysis provides a powerful tool for design optimization and performance prediction.

Manufacturing Considerations

The fabrication of double-skin steel tubes requires specific manufacturing capabilities:

1. Tube production: Both inner and outer tubes must be manufactured to precise dimensional tolerances to ensure concentricity and uniform intertube spacing. The inner tube typically has a smaller diameter and may require cold-drawn or cold-rolled production for dimensional accuracy.
2. Welding: The connection between the inner and outer tubes (if welded) requires careful control of weld quality to avoid distortion that would compromise concentricity. Common welding processes include GTAW (gas tungsten arc welding) for thin-walled tubes and SAW (submerged arc welding) for thicker sections.
3. Concrete placement: The concrete must be placed between the two tubes, which requires specialized placement methods such as pumping through the top of the column with vibration assistance. The intertube space must be completely filled to ensure effective load transfer.
4. Quality control: The concentricity of the double-skin configuration must be verified through dimensional inspection and non-destructive testing (NDT). Ultrasonic testing (UT) can detect voids or incomplete filling in the intertube space.

Design Recommendations

Based on the FE analysis results, the following design recommendations are proposed:

Comparison with Conventional CFST

The DST-HSC column offers several advantages over conventional single-skin CFST columns:

1. Higher load capacity: The dual confinement increases the concrete strength by 30-50% compared to unconfined high-strength concrete.
2. Improved ductility: The dual tube configuration provides a more gradual failure mechanism, with the inner tube failing first and the outer tube continuing to provide confinement.
3. Enhanced fire resistance: The outer tube provides an additional layer of protection against fire exposure, maintaining structural integrity at elevated temperatures.
4. Better corrosion protection: The inner tube is shielded from external corrosion by the outer tube, extending the service life of the column.

Key Questions and Reflections

The FE analysis provides valuable insights into the behavior of DST-HSC columns, but several aspects require further investigation. First, the effect of the intertube medium (air, grout, or concrete) on the confinement efficiency should be systematically studied, as the intertube medium plays a critical role in transferring the confinement pressure from the inner tube to the outer tube. Second, the behavior of DST-HSC columns under combined loading (axial compression with bending or shear) has not been fully explored, which limits the applicability of the findings to practical structural applications. Third, the long-term behavior under sustained loading, including creep and shrinkage effects, should be investigated to assess the durability and long-term performance of the column.

From a steel pipe manufacturing perspective, the DST configuration places demanding requirements on tube production and assembly. The concentricity tolerance, dimensional accuracy, and surface quality of both tubes are critical for achieving the predicted structural performance. Any deviation in these parameters can lead to uneven confinement, localized stress concentrations, and premature failure. Therefore, the manufacturing process must be carefully controlled, with appropriate quality assurance measures at each stage of production.

Summary

This paper presents a rigorous nonlinear FE analysis of double-skin steel tube high-strength concrete axially compressed short columns, demonstrating the significant advantages of the dual-confinement mechanism in enhancing load-bearing capacity, ductility, and fire resistance. The numerical results, validated against experimental data, provide a reliable basis for the design and optimization of DST-HSC columns. For steel pipe manufacturers and structural engineers, the study highlights the potential of advanced CFST configurations for high-performance structural applications, while also emphasizing the critical role of manufacturing quality in achieving the predicted structural behavior. The dual-skin configuration represents a promising direction for future developments in CFST technology, particularly for applications requiring exceptional strength, ductility, and durability.